The invention relates to panel displays and, more particularly, to pixel driving circuitry.
Active matrix organic light emitting diode (AMOLED) displays are currently the prevailing type of flat panel display. As compared with an active matrix liquid crystal display (AMLCD), an AMOLED display typically provides many advantages, such as higher contrast ratio, wider viewing angle, thinner profile, no backlight, lower power consumption and lower cost. Unlike an AMLCD display, which is driven by a voltage source, an AMOLED display requires a current source to drive an electroluminescent (EL) device. The brightness of the EL device is proportional to the current conducted thereby. Variations of the current level tend to impact display uniformity of an AMOLED display. Thus, the quality of a pixel driving circuit, which controls current output, can be critical to display quality.
FIG. 1 illustrates a conventional 2T1C (2 transistors and 1 capacitor) circuit 10 for a pixel in an AMOLED display. When a signal SCAN turns on transistor M1, data (shown as Vdata) is loaded into the gate of P-type transistor M2 and is stored in the capacitor Cst. Thus, a constant current drives the EL device to emit light. Typically, in an AMOLED, a current source is implemented by a P-type thin film transistor (TFT) that is gated by a data voltage Vdata. The source and drain of the P-type TFT are connected to Vdd and to the anode of the electroluminescent (EL) device, respectively. The brightness of the EL device with respect to Vdata therefore has the following relationship:Brightness∝current∝(Vdd−Vdata−Vth)2.